that's all I learned when I took a Japanese class in college :)

There is no
Japanese greeting corresponding to the
English "Hello".

In the morning (till 10 am), you have to say
"Ohayou gozaimasu",which means "good
morning".

In the afternoon(10 am till darkness sets in) , it is "konnichiwa".

This means "good afternoon".
In the evening, it is "kombanwa", which
is "Good Evening".

Hope these help.